DISTE1BUTI0N OE THE MOEE PEOMINENT NATUEAL OEDEES. 267 



But it is at elevations of 6000 to 10,000 feet that the enormous pine-forests of 

 Mexico chiefly exist*. For fuller details of the distribution of the Coniferge the 

 reader is referred to the writings of Beinling, Brown, and Hildebrand enumerated in 

 our Bibliography. 



Cycadacece. 

 The number of described genera and species of this order affords only a very inadequate 

 idea of the prominent feature cycads form in the vegetation of the districts where they 

 grow; and as they are not readily reduced to herbarium specimens the particulars 

 usually accompanying such are to a great extent wanting for this order. Nine genera 

 and about seventy-five species are known, many of them imperfectly, and they are 

 thinly scattered over the tropics, South Africa, and Australia ; and Cycas revoluta is a 

 native of Japan. No species has been found in the dry regions of Chili. The genera 

 of the Old World and America are all different, and the majority of the species are 

 local ; Cycas circinalis is, however, an exception, being widely spread in the tropics of 

 the Old "World, especially in littoral districts. Within our districts there are three 

 genera, two of them endemic in Mexico, and one general in tropical America ; and the 

 number of species is twenty-one, whereof fifteen are endemic ; three extend to South 

 America, and three to the West Indies. Altogether sixteen species are recorded from 

 Mexico, but several of them are doubtful, and more complete material is needed of 

 almost all of them. Stangeria in South Africa and Bowenia in Eastern Australia are 

 very remarkable monotypic genera of this order. 



Orchidece. 



Orchids are generally diffused in all regions, and very nearly reach the extreme 

 altitudinal and latitudinal limits of phanerogamic plants ; yet they are either absent or 

 exceedingly rare in oceanic islands f . With the exception of the terrestrial genera of 

 north temperate regions, and a few others, the genera are mostly restricted to the 

 eastern or western hemisphere, and largely to single continents. This holds good both 

 for tropical genera and the terrestrial genera of the southern hemisphere, where totally 

 different genera inhabit the three great areas. The same remarks are applicable to 

 species in a greater degree, a large proportion of which are comparatively local. The 

 greatest concentration of orchids is in America, from South Mexico to Colombia. 



Mexico and Central America are pre-eminently rich, and, although Nicaragua and 

 Costa Rica have been very little explored, and many of the small-flowered Mexican 

 ones doubtless overlooked, yet orchids have probably been quite as fully collected as 

 any class of plants ; therefore future investigations are likely to lower rather than raise 

 the proportion of orchids to the rest of the flora. Orchidese in Mexico, so far as our 

 present knowledge goes, stands third in the list of orders according to number of 



* C. B. Heller, Eeisen in Mexico, pp. 171, 182, &e. 



t See Voyage of the ' Challenger ' Expedition, Botany, i. Introd. p. 27. 



biol. CENTE.-AMEE., Bot. Vol. IV., August 1887. 2 » 



